With five centuries of hindsight, it seems that the development of mechanical theories proceeded predictably. In the year 1600, however, it was by no means clear where to begin, what to study or what questions to ask. One of the key items on the physics agenda was change. The ancient Greek philosophical dictum, panta rhei (‘everything flows’), lacked precision and was unacceptable in experimental philosophy as an explanation for change.

Change presents itself in many ways, the most obvious being the motion of objects. In 1600, common sense seemed to show that motion needs an agent to produce and to maintain it, and everything seemed to move towards the ground, unless something interfered. The first person in the history of our planet to make serious headway on this subject was Galileo. He did this by replacing the philosophical question, ‘What is motion?’ with the experimental-philosophical inquiry, ‘How does motion behave?’

With his immense perception and talent for precise and systematic experimentation, Galileo began to study the behaviour of spherical balls that were set up to roll down inclined planes so that they moved more slowly than simply dropping vertically. In a long series of experiments, he discovered most of the basics of falling motion.

First: the velocity increases in direct proportion to time (in the absence of perturbations such as the resistance of the air). That is to say: free fall is uniformly accelerated motion. Second: the speed that an object acquires when released is always the same after it has fallen a given vertical distance, in free fall as well as when constrained to move on an inclined plane. Third: Galileo deduced and verified that it follows from the first finding (speed is proportional to time) that the distance an object traverses when falling is proportional to the square of the time. Fourth: horizontal and vertical motions occur independently. From this, he concluded that the path of a thrown mass is a parabola.

In parallel with these experiments, Galileo developed a view of the more general and abstract properties of motion. The consensus at the time was that motion requires something to keep it going, with the exception of motion ‘above the Moon’, that is, the motions of the planets.